The present invention relates to bipolar logic circuits and, more particularly, to a transient driver circuit for use in conjunction with monolithic bipolar logic circuits which provides an output transient charge and discharge current path.
Historically, one of the most important advantages of bipolar transistor technology has been its ability to drive large resistive and capacitive loads with relatively small device geometries. For instance, a typical emitter coupled logic (ECL) gate comprises an emitter coupled logic stage coupled to a transistor emitter-follower output stage which restores DC levels. The emitter-follower output stage sources output current at the output of the ECL gate that is an exponential function of the emitter-base voltage of the output transistor. High fan-out operation is possible because of the high input impedance of the emitter coupled logic stage and the low output impedance of the emitter-follower output stage. To minimize quiescent power dissipation in the ECL gate, the emitter-follower output stage typically includes a resistor coupled between the output of the ECL gate and the negative power supply rail which sets the quiescent current in the emitter-follower output stage. A problem occurs when such prior art ECL gates are operated into large capacitive loads which must be discharged as the output of the gate switches from a first logic level to a second logic level. In prior art ECL gates the discharge current must flow through the resistor to the negative supply rail of the gate. Since the resistor is much larger in value than the "on" resistance of the emitter-follower transistor, discharge of the capacitor is much slower than the charge thereof since discharge is at a near constant rate determined by the resistor. Hence, the input logic signal aplied to the ECL gate is slewed, i.e., the resultant fall time of the output signal is different than the input signal. This is highly undesirable.
To decrease the discharge time the resistance value of the resistor coupled to the emitter follower stage of the ECL gate can be made smaller. However, this results in a undesirable increase in quiescent power dissipation since the quiescent current flowing through the emitter-follower output stage is increased. In order to be able to utilize bipolar ECL gates in complex logic circuits requires reducing the static power requirement of the ECL gate without sacrificing speed. In addition to the bipolar ECL gates must be able to provide transient source and sink currents such that large capacitive loads can be driven at high speed while at the same time minimizing the quiescent power dissipation of the ECL gates.
Hence, a need exists for a transient driver circuit for use in conjunction with bipolar logic circuits such as ECL gates which can quickly discharge as well as charge capacitive loads coupled to the output of logic circuits by providing a transient discharge current only during transient switching conditions.